PGS stands for preimplantation genetic screening, a test performed on embryos during IVF to check whether they have the correct number of chromosomes before being transferred to the uterus. The term PGS has been officially replaced by PGT-A (preimplantation genetic testing for aneuploidy), though many clinics and patients still use the older name. The goal is straightforward: identify embryos with the best chance of leading to a healthy pregnancy and avoid transferring ones with chromosomal errors that would likely result in failed implantation or miscarriage.
What PGT-A Actually Tests For
Human cells normally contain 46 chromosomes, arranged in 23 pairs. Aneuploidy means an embryo has too many or too few chromosomes, and it’s the single most common reason embryos fail to implant or pregnancies end in miscarriage. PGT-A screens all 23 chromosome pairs at once, looking for missing or extra copies. The most well-known chromosomal conditions it can detect include trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome), but it also catches errors across every other chromosome.
Modern testing platforms can also identify triploidy (an entire extra set of chromosomes) and uniparental disomy, where both copies of a chromosome come from the same parent instead of one from each. Older methods like FISH could only examine a handful of chromosomes at a time, which is why the field moved to newer technologies that analyze all 24 chromosome types (22 numbered pairs plus X and Y) simultaneously.
How PGT-A Differs From PGD
You’ll sometimes see the term PGD alongside PGS, and the distinction matters. PGS (now PGT-A) screens for random chromosomal number errors, the kind that become more common as egg quality declines with age. PGD (now split into PGT-M and PGT-SR) is different: it looks for specific known genetic conditions. PGT-M tests for single-gene disorders like cystic fibrosis or sickle cell disease when one or both parents are carriers. PGT-SR tests for structural chromosome rearrangements like translocations or inversions, typically when a parent is known to carry one. PGT-A is a broad screening tool. PGT-M and PGT-SR are targeted diagnostic tests.
How the Biopsy Works
The testing requires removing a small number of cells from each embryo, a procedure called a trophectoderm biopsy. This happens on day 5 or 6 after fertilization, when the embryo has reached the blastocyst stage and contains over 100 cells. A laser opens a small section of the embryo’s outer shell, and the embryologist carefully removes 5 to 10 cells from the trophectoderm, the outer layer that will eventually become the placenta rather than the baby itself. Seven to eight cells is considered the ideal sample size for reliable genetic analysis.
After the biopsy, embryos are frozen while the cell samples are sent to a genetics lab. Results typically come back within one to two weeks, and the embryo transfer happens in a subsequent cycle. This freeze-all approach is now standard practice for PGT-A cycles. Early versions of the technology biopsied embryos at day 3, when they had only about 8 cells total, which meant removing one or two cells amounted to roughly 25% of the embryo’s mass. That older approach caused a significant drop in implantation rates, around 39% lower compared to unbiopsied embryos. Day-5 biopsy avoids this problem because the embryo is much larger and the cells removed come from the placental layer.
How Accurate Is the Test
PGT-A is highly accurate but not perfect. A large meta-analysis of diagnostic accuracy found that trophectoderm biopsy achieves a sensitivity of about 84% and specificity of about 79%. That means the test correctly identifies most abnormal embryos, but there is a meaningful false positive rate: some embryos labeled abnormal may actually have been capable of producing a healthy pregnancy. This is one of the more important limitations to understand, because a false positive result means a viable embryo could be discarded.
One reason for these imperfect numbers is mosaicism. A mosaic embryo contains a mix of chromosomally normal and abnormal cells. Since PGT-A only samples a few cells from the outer layer, the biopsy might not represent what’s happening throughout the entire embryo. Mosaicism is detected when the DNA analysis shows an intermediate chromosome copy number, not clearly normal and not clearly abnormal. The reproductive medicine community has gradually moved toward accepting embryos with low-level mosaicism (generally under 50% abnormal cells) as viable transfer candidates, since preliminary evidence suggests these levels may not significantly impact early development.
Impact on Pregnancy Outcomes
The clearest benefit of PGT-A shows up in miscarriage reduction. For patients with recurrent pregnancy loss, the testing cuts miscarriage rates per transfer roughly in half and more than doubles the live birth rate per transfer. That’s a significant difference for people who have experienced repeated losses.
The picture is more nuanced when it comes to overall live birth rates for the general IVF population, and age plays a central role. A large analysis using data from the Society for Assisted Reproductive Technology found that for patients under 35, PGT-A was actually associated with slightly lower cumulative live birth rates (70.6% with testing versus 71.1% without). For patients aged 35 to 37, there was no meaningful difference (66.6% versus 62.5%). This doesn’t mean PGT-A is harmful for younger patients. It likely reflects the fact that younger patients produce more chromosomally normal embryos to begin with, so the testing process (with its added cost, freezing, and occasional false positives) doesn’t improve their overall odds. The benefits become more pronounced for patients over 37 or 38, when aneuploidy rates climb steeply and selecting the right embryo matters more.
What PGT-A reliably does across all age groups is reduce the time to pregnancy for patients who would otherwise transfer aneuploid embryos that fail or miscarry. Instead of going through multiple unsuccessful transfers, you transfer an embryo that has already passed a chromosomal screen.
What It Costs
PGT-A adds a meaningful expense on top of an already costly IVF cycle. The average cost of testing runs around $4,300, though it can range from roughly $3,000 to over $12,000 depending on the lab, the number of embryos tested, and the testing platform used. Some labs charge a flat fee per cycle while others charge per embryo, so the total depends heavily on how many embryos you produce. When combined with the base cost of IVF (averaging around $18,000 per cycle), the total bill for a PGT-A cycle often lands between $22,000 and $30,000. Insurance coverage for genetic testing varies widely and many plans do not cover it at all.
Who Typically Uses PGT-A
PGT-A is most commonly recommended for patients over 35 to 37, since the rate of chromosomally abnormal embryos rises sharply with age. By age 40, more than half of embryos are typically aneuploid, and by 43 or 44 that number can exceed 80%. Other common candidates include couples who have experienced recurrent miscarriage, those with repeated failed embryo transfers, and patients who want to minimize the chance of a multiple pregnancy by feeling confident transferring a single embryo.
Some clinics recommend PGT-A for nearly all IVF patients regardless of age, while others take a more selective approach. Given the data showing limited benefit for younger patients in terms of cumulative live birth rates, the decision often comes down to individual circumstances: how many embryos you have, your history, your tolerance for the possibility of a failed transfer or miscarriage, and whether the added cost fits your financial situation. There is no universal right answer, and it’s worth understanding both the strengths and the real limitations of the technology before deciding.